Relazioni tra struttura e proprietà nelle malte storiche romane. Una lezione dal passato e un tentativo di proiezione nella moderna scienza dei materiali

The present work fits into the scientific and technological context committed to explain the impressive durability of ancient Roman mortars. Starting from available literature, the study has been expressly designed to provide a deeper insight into the mechanisms, not yet fully understood, responsible for the formation of the observed mortar structure, and the related development of physical and chemical properties. In particular, experimental investigation has focused on the factors governing the reactivity of interfaces during the reaction between pozzolanas and quicklime, and the formation of mineralogical phases assuring cohesion to the material. It is also shown that the application of modern materials science methodologies to the fabrication of traditional mortars bears significant innovation potential for building and restoration practices. The research drew inspiration from most recent claims of scientific literature identifying the structure of historic mortars as the ultimate reason of its durability. Emphasis is definitely laid on the need of understanding the relationship between the mortar microstructure and the reactivity of its constituents. Experimental activity started with an accurate study of an original fragment of Roman opus caementicium from Trajan’s Markets, in Rome. Such study was rapidly supported with the fabrication of mortars according to classical recipes using pozzolanic materials collected from Roman ancient quarries with the aim of investigating the physical and chemical processes connected with the aging of the material. Along with the above-mentioned investigation, the effects of chemical reactivity on aging were studied. To this aim, the pozzolanic material, both pure and mixed with lime, was subjected to mechanical activation in the absence and in the presence of water. Within this framework, the chemical reactivity of pozzolanas was evaluated measuring the density of radical species generated on the surface of powder particles consequent to impulsive loading and frictional processes taking place during the mechanical treatment. Mechanical activation in the absence of water enhances the reactivity of pozzolana-lime mixtures, which results in a faster chemical conversion to the products of pozzolanic reaction once water is added to the processed powder. In the presence of water, mechanical processing determines a significant increase of the reaction rate, inducing a significant acceleration of hardening.